1. Field of the Invention
The present invention relates to a cross-coil type measuring instrument which is employed as an engine tachometer for instance.
2. Description of the Related Art
A conventional cross-coil type measuring instrument, as shown in FIG. 9, comprises a frequency measuring circuit 11 for measuring either frequency or period of a pulse signal produced by a sensor such as a vehicle running speed sensor, an arithmetic circuit 13 for dividing an output of the frequency measuring circuit 11 into a sine wave component and a cosine wave component, a since wave function generator 21 and a cosine wave function generator 22, a first pulse wave modulation circuit 14 for subjecting an approximate sine wave signal from the sine wave function generator 21, a second pulse wave modulation circuit 15 for subjecting an approximate cosine wave signal from the cosine wave function generator 22, and output circuits 31 and 32 for driving coils 31 and 32 of a cross-coil type measuring unit 4, respectively, in response to the outputs of the first and second PWM circuits 14 and 15. Reference numeral 7 designates a constant voltage source connected to a battery 6 to produce a constant voltage output of 8 V. Published Unexamined Japanese Utility Model Application No. Hei-3-74363 discloses an example of the above described conventional instrument.
The conventional cross-coil type measuring instrument thus organized suffers from the following difficulties. That is, the instrument has a mechanical hysteresis attributing to its bearings or the like. In order to cancel the hysteresis, a constant voltage of 8 V which is higher than an ordinary power source voltage is applied to the drive circuits 24 and 25 so that predetermined currents are caused to flow in the cross coils 31 and 32 to drive a cross-coil type measuring unit 4. As a result, the measuring unit 4 is allowed to indicae a correct measurement value. Hence, not only the constant voltage circuit 7 but also the cross coils 31 and 32 generate a large amount of heat. Accordingly, it is necessary for the instrument to have heat radiating means. In other words, the instrument must have a space for installation of a heat radiating board.
Furthermore, with the conventional cross-coil type measuring instrument, the coils 31 and 32 may be different in length, and accordingly in resistance, which leads to an error in indication. In addition, since the constant voltage source 7 and the coils 31 and 32 produces a large amount of heat as was described above, the coils are changed greatly in resistance, which also leads to an error in indication.
Moreover, with the conventional cross-coil type measuring instrument, the drive coils 31 and 32 are arranged crossed, forming 90.degree. degree with each other. Therefore, when a current flows in one of the drive coils 31 and 32, which attributes to a square wave coil drive voltage concerning the PWM (pulse width modulation) signal, then a mutual interference occurs between the drive coils 31 and 32, for instance, by electromagnetic induction, so that an induction current flows in the other drive coil (hereinafter referred to as "an interference current", when applicable). The interference current is superposed on the coil drive signal, thus resulting in an error in indication. The value of the interference current depends on the value of the coil drive current and the electromagnetic coupling condition of the drive coils 31 and 32. Hence, in the case of the drive coils 31 and 32 in which the coil drive current and the magnetic coupling condition change with the ambient temperature, the value of the induction current changes with the ambient temperature. Therefore, even if the indication is adjusted at room temperature, the indication will be deviated depending on the ambient temperature. In this case, it is rather difficult to process the signals suitably by using circuit elements.